1. Field of the Invention
The present invention relates to a backlight unit for a liquid crystal display (LCD) device, and more particularly to a backlight unit for an LCD device that effectively reduces an induction current by generated by parasitic capacitance by inserting a resistor in a reflection film.
2. Discussion of the Related Art
In the current information society, the roles of electronic display devices become increasingly important, such that various types of electronic display devices are used in the diverse industrial sectors. Accordingly, the electronic display devices having new functions are continuously developed to meet the demands of the information society. In general, an electronic display device refers to a device for visually presenting the information to humans. Specifically, the electronic display device converts electronic information signals output from the various electronic apparatus into optical information signals recognizable by humans. Thus, the electronic display device serves as a mediator between humans and the electronic apparatus.
There are two types of display devices: a light emission type and a light receiving type. The light emission type display device displays light information signal according to an illumination phenomenon. The light emission type display device is also called an active display device and includes a cathode ray tube (CRT), a plasma display panel (PDP), an organic electroluminescent display (OELD), a light emitting diode (LED), and the like. The light receiving type display device displays light information signal that has been light-converted according to a reflection, scattering, and interference phenomenon. The light receiving type display apparatus is also called a passive display device and includes the LCD, an electrophoretic image display (EPID), and the like.
The CRT has been employed for televisions and/or computer monitors and still occupies a high market share in terms of economical efficiency. However, due to its disadvantages, such as heavy weight, bulky size, and higher power consumption, the CRT is facing challenges competing against the flat panel display devices. Recently, the rapid advancement of the semiconductor techniques allows various electronic devices to improve their power consumption, their sizes, and their weights. Accordingly, the flat panel displays devices such as the LCD, the PDP and the OELD devices have been developed. Specifically, the LCD devices are developed having small and thin size, light weight, low power consumption, and low drive voltage.
Since the LCD devices are light receiving type display devices, they need a light source such as a backlight unit. There are two types of backlight units for the LCD devices: a direct type backlight unit and a light guide plate type backlight unit. The direct type backlight unit is constructed such that a plurality of lamps are arranged on a plane and a diffuser is inserted between the lamps and a liquid crystal panel to maintain a space between the liquid crystal panel and the lamps. The light guide plate type backlight unit is constructed such that lamps are installed at an outer edge of a flat panel and light received from the lamps is irradiated onto the liquid crystal panel by using a transparent light guide plate. The direct type backlight unit is commonly used for the LCD device because it has a high light utilization rate and can be easily handled.
A related art backlight unit for an LCD device will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view showing parasitic capacitance formed between lamps of a related art backlight unit for an LCD device and a bottom cover. FIG. 2 is a view showing waveforms of a high level voltage applied to the lamps and an induction current generated by the parasitic capacitances between the lamps and the bottom cover.
As shown in FIG. 1, the related art backlight unit for an LCD device comprises a plurality of lamps 13 arranged in parallel with each other to irradiate light, a bottom cover 15 for receiving the lamps 13, and a reflection film 14 positioned between the bottom cover 15 and the lamps 13. Each of the lamps 13 includes inert gases (Ar and Ne) inside a glass tube, and a cathode and an anode installed at end portions of the glass tube. The glass tube filled with the inert gases therein has phosphor coating on an inner wall of the glass tube.
When an AC voltage is applied to the anode and the cathode of each of the plurality of lamps 13 from an inverter (not shown), electrons are discharged from the cathode to collide with the inert gases inside the glass tube. The amount of electrons increases by geometric progression. When a current flows inside the glass tube owing to the increased electrons, the inert gases are excited by the electrons, thus generating energy that excites the molecules to emit ultraviolet rays. The ultraviolet rays collide with illuminative phosphor that is coated on the inner wall of the glass tube, thereby emitting visible light.
The bottom cover 15 is made of a metallic material. As shown in FIG. 1, parasitic capacitance C1, C2, and C3 are generated between the lamps 13 and between the lamps 13 and the reflection film 14/bottom cover 15. Specifically, the parasitic capacitance C1 is formed between the lamp 13 and one side of the bottom cover 15, the parasitic capacitance C2 is formed between the lamps 13, and the parasitic capacitance C3 is formed between the lamps 13 and the lower bottom cover 15. As shown in FIG. 2, when a high level voltage is applied to drive the plurality lamps 13, an induction current is generated by the parasitic capacitances C1, C2, and C3. Since the induction current is induced by the parasitic capacitances C1, C2, and C3, it is noted that the phase difference between the induction current and the high level voltage is π/2(90°). The induction current does not go to driving of the plurality of lamps 13, thus decreasing illumination efficiency of the lamps 13.